The goal of this study is to understand the molecular basis of myelin's inhibitory effect on axonal regeneration. The specific hypothesis to be tested is whether ephrinB3, a known axon repellant during development, is a myelin-based inhibitor with respect to the regeneration of injured corticospinal tract (CST) axons that express the EphA4 receptor. Experiments in Aim 1 will employ an in vitro assay deteremine if ephrinB3 is, in fact, inhibitory to neurite outgrowth from postnatal cortical neurons, its potency as an inhibitor compared to other known myelin-based inhibitors and the contribution it makes to the overall inhibitory activity of myelin. Aim 2 will use site directed mutants of the EphA4 receptor tyrosine kinase (the receptor for ephrinB3 in CST development) in a neuronal cell culture system to determine which amino acids in the intracellular portion of the molecule are required for ephrin-induced repulsion/inhibition. Results from these experiments will, hopefully, lead to the identification of key signalling molecules involved in inhibition by ephrins of axonal regeneration. Studies in Aim 3 will seek to extend the findings of experiments in Aim 1 by examining a mouse model of chronic spinal cord injury coupled with genetic and pharmicological blockage of Eph/ephrin function.